Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 282016
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
316 PM CDT FRI AUG 28 2015

.SHORT TERM:  (THROUGH LATE TONIGHT)
ISSUED AT 315 PM CDT FRI AUG 28 2015

WATER VAPOR IMAGERY SHOWS THAT A SHORTWAVE TROUGH IS MOVING SLOWLY
EAST ACROSS MISSOURI AT THIS HOUR.  LATEST RUNS OF THE RAP SUGGEST
THAT IT WILL TAKE ALL EVENING FOR THIS FIRST SHORTWAVE TO REACH
HALFWAY ACROSS THE CWA WHILE A SECOND SHORTWAVE WILL ENTER WESTERN
MISSOURI BEHIND IT LATER TONIGHT.  LOW LEVEL MOISTURE CONVERGENCE
WILL INCREASE ACROSS EASTERN MISSOURI BY LATER THIS EVENING AS LOW
LEVEL JET VEERS AND INCREASES IN SPEED SLIGHTLY.  THIS SHOULD CAUSE
AN INCREASE IN COVERAGE IN SCATTERED SHOWERS AND THUNDERSTORMS BY
LATE EVENING AND OVERNIGHT OVER THE REGION BECAUSE OF THE FORCING
FROM THE SHORTWAVES AND THE MOISTURE CONVERGENCE.  THE 1KM
REFLECTIVITY FROM THE CAMS DEPICTS THIS PRETTY WELL.

MOS LOWS TONIGHT LOOK REASONABLE GIVEN THE INCREASING DEWPOINTS
AHEAD OF THE COLD FRONT CURRENTLY OVER THE CENTRAL PLAINS.

BRITT

.LONG TERM:  (TONIGHT THROUGH THURSDAY)
ISSUED AT 340 AM CDT FRI AUG 28 2015

THERE IS GOOD AGREEMENT IN THE 00Z GUIDANCE THAT FRONTAL
BOUNDARY AND ASSOCIATED SHORTWAVE SHOULD ALLOW THUNDERSTORMS
THREAT TO SLOWLY EXPAND OVER THE NORTHWEST HALF OF THE CWA
TONIGHT...WITH THE STL METRO LIKELY ON THE EASTERN EDGE OF THE
THREAT DURING THE PREDAWN HOURS.  DUE TO PROGGED LOW LEVEL
FORCING AHEAD OF THE FRONT, HAVE UPPED POPS INTO LIKELY CATEGORY
OVER OUR EXTREME NORTHWEST COUNTIES.

HOWEVER...FROM SATURDAY MORNING ON FEATURES AND MODEL SPECIFICS
BEGIN TO DIVERGE AND BECOME INCREASING NEBULOUS.  WHILE MODELS DO
AGREE THAT NORTHERN PORTION OF THE SHORTWAVE WILL CONTINUE TO PRESS
INTO THE LOWER GREAT LAKES DURING THE DAY...THEY DISAGREE WITH THEIR
HANDLING OF THE SOUTHERN PORTION OF THE SHORTWAVE ENERGY AS IT
STRETCHES SOUTHEAST AND POSSIBLY MERGING WITH ANOTHER WEAK
CIRCULATION ALONG THE GULF COAST (WHICH APPEARS TO ORIGINATE WITH
A BIT OF ENERGY THAT BREAKS OFF FROM THE BASE OF THE CURRENT EAST
COAST TROF). GFS AND NAM SUGGEST SOME TYPE OF REMNANT CIRCULATION
TAKING UP RESIDENCE IN THE VICINITY OF SW MO BY SUNDAY WITH
RESIDUAL SURFACE FRONT BECOMING STATIONARY OVER THE AREA...WHILE
ECMWF WANTS TO PUSH THE INCREASINGLY WEAK SHORTWAVE ENERGY AND
SURFACE BOUNDARY SOUTH WITH TIME.

IF NAM AND GFS SOLUTIONS DO COME TO FRUITION ON SUNDAY...AND IF NAM
LOW LEVEL MOISTURE FORECAST (WHICH IS OFTEN TOO HIGH) ALSO
VERIFIES...THEN THE DRY FORECAST ON SUNDAY WILL BE IN JEOPARDY.
HOWEVER...WOULD LIKE TO SEE ADDITIONAL SUPPORT FOR THESE WEAK
FEATURES AND TRENDS BEFORE REINTRODUCING CONVECTION ON SUNDAY.  SO
FOR NOW HAVE CONTINUED TO TREND THE FORECAST TO THE ECMWF
SOLUTION...NAMELY THUNDERSTORM CHANCES EXPANDING SOUTHEAST ACROSS
THE CWA DURING THE DAY ON SATURDAY THEN GRADUALLY DIMINISHING FROM
NW TO SE LATE SATURDAY AFTERNOON AND SATURDAY NIGHT.

DESPITE THE DISCREPANCY IN THE UA PROGS ON SUNDAY...MEDIUM RANGE
SOLUTIONS REMAIN IN GOOD RUN TO RUN AND MODEL TO MODEL AGREEMENT
THAT WESTERN RIDGE WILL BE EXPANDING EAST INTO THE MID
MISSISSIPPI AND OHIO VALLEYS DURING THE UPCOMING WEEK. THIS SHOULD
MEAN DRY WEATHER FOR THE FA...ALONG WITH TEMPERATURES REBOUNDING
TO LATE SUMMER LEVELS WITH HIGHS IN THE UPPER 80S-LOWER 90S.

TRUETT

&&

.AVIATION:  (FOR THE 18Z TAFS THROUGH 18Z SATURDAY AFTERNOON)
ISSUED AT 1237 PM CDT FRI AUG 28 2015

EXPECT MAINLY DRY AND VFR CONDITIONS THROUGH THIS EVENING. THEN
EXPECT SCATTERED SHOWERS AND THUNDERSTORMS TO DEVELOP ACROSS
CENTRAL AND NORTHEAST MISSOURI AND WEST CENTRAL ILLINOIS OVERNIGHT
THAT WILL EVENTUALLY SPREAD INTO THE ST. LOUIS METRO AREA ON
SATURDAY MORNING. SCATTERED THUNDERSTORMS WILL CONTINUE TO DEVELOP
ON SATURDAY AFTERNOON AROUND THE ST. LOUIS METRO TAF SITES.

SPECIFICS FOR KSTL: EXPECT MAINLY DRY AND VFR CONDITIONS THROUGH
TONIGHT. THEN SCATTERED SHOWERS WILL MOVE INTO THE AREA BY LATE
SATURDAY MORNING WITH AN INCREASING CHANCE OF SCATTERED
THUNDERSTORMS AS AN UPPER LEVEL STORM SYSTEM MOVES INTO THE AREA
BY LATE SATURDAY AFTERNOON.

BRITT

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX



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